SOME PRELIMINARY REMARKS ON THE VELOCITY OF MIGRATORY FLIGHT AMONG BIRDS, WITH SPECIAL REFERENCE TO THE PALEARCTIC REGION.1

By Col. R. MEinertzhagen, D. S. O., M. B. O. U., F. Z. S.

The question arises at once as to whether migratory flight is of a different nature from daily flight in search of food or to escape enemies. We have some interesting opinions on this subject. Gätke tells us that the speed of birds during their daily locomotions in the air has not an approximate relation to the wonderful velocity of flight attained by them during their migrations. He accounts for such enormous speed by the fact that birds migrate in the more elevated layers of the atmosphere, in which more uniform conditions prevail, and which are less subject to powerful meteorological disturbances.

Cooke ("Bird Migration "), on the other hand, thinks that migrating birds do not fly at their fastest. He believes that their migrating speed is usually from 30 to 40 miles an hour, and rarely exceeds 50. Flights of a few hours at night, alternating with rests of one or more days, make the spring advance very slow. He goes on to say that during day migration the smaller land birds seldom fly faster than 20 miles per hour, though larger birds move somewhat more rapidly.

I believe Gätke's theory to be based on faulty evidence, as I hope to show later. Moreover, birds would experience greater difficulties in flying in the "more elevated layers of the atmosphere," as the atmosphere is rarer and therefore offers a less suitable mixture on which their wings can beat. They would experience the same difficulties as a man trying to swim in froth.

My own observations tend to show that migratory flight differs very little in its velocity from the flight of daily movement, and I see no reason why it should or how it can be so. I believe migratory flight to be steady and unhurried, and that birds only fly at their fastest when pursuing or when pursued. Any one who has watched a falcon being flown at a rook will be struck by the speed which the

1 Reprinted by permission from The Ibis, April, 1921, pp. 228-238.

usually leisurely flapping rook can attain from the moment he realizes he is the quarry.

I have seen rooks traveling on migration, and accurate observation gives their pace as from 38 to 40 miles per hour. Now these migratory rooks were traveling in their usual leisurely fashion, and not at anything like the speed they can use when attacked by a falcon. All other migrations which I have witnessed in many and various parts of the world confirm my belief that migratory flight differs in no way from every-day movement, except that it is steadier and possibly a trifle slower.

So in dealing with this question, I shall consider estimates of any normal flight as the normal velocity which birds attain on migration. That birds can hurry I do not doubt, but such effort could not be long sustained, and would be of little use to them in the long-distance migratory journeys they are accustomed to take.

I shall first deal with those estimates of velocity which previous writers have recorded, but which can not be regarded as reliable. Gätke claims that hooded crows fly at 108 miles per hour and bluethroats at 180 whilst on passage, and especially in the spring. He claims that bluethroats pass from between 10° and 27° of northern latitude to the fifty-fourth degree of northern latitude in 9 hours. He also assumes that the American golden plover takes but 15 hours from Labrador to northern Brazil, supporting this theory by his personal observations on godwit and curlew covering over 7,000 yards in 60 seconds, or at the rate of over 4 miles a minute!

His estimate of hooded crow flight is based on the assumption that their line of flight is from east to west over Heligoland, and that they make for the east coast of England. This apparently is not the case, for their line of autumnal flight over Heligoland is from northeast to southwest, and these are probably not the birds which arrive in such numbers on our central east coast. The bluethroat estimate is based on the assumption that birds fly direct from Egypt to Heligoland in one night, which is certainly not the case. estimate of the flight of godwit and curlew, on which he bases his estimate of the flight of the American golden plover, is, I fear, but an example of the tremendous enthusiasm of this charming character for his subject.

His

But Gätke is not alone in overestimating the velocity of flight. Many other writers have erred through basing a theory on bad evidence or no evidence at all, one of the most remarkable of these being Crawfurd ("Round the Calendar in Portugal "), who convinced himself that turtledoves flew at such an astonishing pace that by leaving Kent at dawn they would be in Portugal a few hours later.

As regards more accurate data, it was my fortune during the recent war to have the opportunity of using antiaircraft arrangements for my purpose. It was excellent practice for the men, and the results. can be taken as accurate for all practical purposes. In conjunction with observations of an accurate nature from other sources, I have compiled the following table.

Unless the authority is stated in parentheses, the observations are my own.

The following notes refer to the table:

NOTE A.-Observations taken at Quetta by two persons with stop watches over a measured distance varying from 400 to 600 yards. All birds were below 1,000 feet, and in no case were they migrating.

NOTE B.-Observations taken in East Africa in the autumn of 1915 on migrants by using theodolites on a base of 1,200 feet.

NOTE C.-Observations made at Dar-es-Salaam by a system of two persons with stop watches stationed 440 yards apart and timing birds flying between points aligned by two stakes. All observations taken on still evenings when birds were flying to and from their breeding grounds.

NOTE D.-Observations made near Rafa, in southern Palestine, during the autumn of 1917 by means of theodolites at two antiaircraft gun stations on a base of 3,926 feet, the stations being connected by telephone.

NOTE E.-Observations taken in southern Palestine by stop watches at 440 yards distance and timing birds flying between two points aligned by posts.

NOTE F.-Observations made near Montreuil, in northeast France, by means of theodolites on a 1,420-foot base and small balloons to ascertain the velocity of the wind at the altitude of flight. All birds believed to be on migration.